1. Field of the Invention
The present invention relates to an apparatus for converting pulse code modulation (PCM) signal, and more particularly, the present invention relates to a PCM signal conversion apparatus capable of providing compatibility between two different coding standards, namely, A-law and μ-law in a communication system.
2. Description of the Related Art
A device typically found in digital communication systems is a converter for converting an analog signal to a digital signal. Typically, PCM technique is implemented for converting an analog signal to a digital signal. The PCM signal contains a plurality of groups of multi-bit digital signals constituting a series of multi channel signals. The PCM technique can be classified into two types of coding, A-law PCM coding (simply called A-law coding herein after), which is widely used in Europe, and μ-law PCM coding (simply called μ-law coding hereinafter), which is widely used in US and Japan.
Currently, the A-law coding and the μ-law coding are both widely used in many countries, as a result, there is a demand for a separate device for interfacing between the A-law coding and the μ-law coding.
FIG. 1 illustrates a conventional code conversion circuit of the type having a ROM (read only memory) for receiving at its address input, a digital trunk card including a time switch/digital signal processor (DSP) (called a time switch/DSP hereinafter), a main control card including a main time switch, and a digital trunk, and the circuit used for converting the signals encoded by different coding standards.
Referring to FIG. 1, the system employs the μ-law coding, wherein signals B and D represent A-law modulated (or encoded) signals and signals A and C represent μ-law modulated signals. A digital signal from an office line is supplied to the main time switch 112, passing through a digital trunk 124 and a time switch/DSP 122. The time switch/DSP 122 converts the A-law modulated signal B into the μ-law modulated signal A. The time switch/DSP 122 first receives the A-law modulated code value B and outputs a stored μ-law modulated value corresponding to the received A-law modulated signal B as linear code value, and then replaces the linear code with corresponding μ-law modulated value from an pre-determined internal memory table. Accordingly, the A-law modulated signal B is converted into the μ-law modulated signal A and subsequently transmitted to a main control card 110.
In contrast, when a signal is output to the office line from the system, the μ-law modulated signal C output from the main control card 110 is applied to the time switch/DSP 122. The time switch/DSP 122 then converts the μ-law modulated signal C into corresponding A-law modulated signal D from the pre-determined internal memory table using the above process in reverse order. As a result, the converted A-law modulated signal D is supplied to the office line via the digital trunk 124.
All the signals, A, B, C and D, as shown in the FIG. 1, are digital signals (or PCM signals). The signals A and C represent the μ-law modulated signals while the signals B and D represent the A-law modulated signals. As shown, the above known converter indiscriminately converts every channel signal in the multi-channel signals. However, it has the some disadvantages in operating that way. First, the design of the above conversion apparatus is complicated, which results in a high fabrication cost and complicates the overall size of the system. Moreover, since all the channel signals of the multi-channel signals are converted indiscriminately, it is not suitable for selectively converting only one or two channel signals of the multi channel signals.